Preheat oven to 355°F (180°C). Put liners into a 12 hole regular sized cupcake tin. Set aside.

Bloom the cocoa by combining cocoa powder and hot water (or hot coffee) in a medium bowl and whisk until smooth. Set aside to cool while you make the batter.

In a large bowl, whisk together egg, brown sugar, granulated sugar, salt, and vanilla until it lightens in color.

Add buttermilk and oil into the batter, whisk to combine.

Sift flour, baking powder, and baking soda right into the bowl. Fold with a whisk until just combined, using the whisk to break up lumps. Do not overmix.

Add bloomed cocoa powder into the batter. Use a whisk to fold gently until even.

Scoop batter into the lined muffin tin half of the way full. Press an unwrapped Ferrero Rocher into each muffin tin. Cover each chocolate with a teaspoon of extra batter so that the tins are slightly over ¾ of the way full. Bake for 16-18 minutes or until a toothpick inserted into the middle (not all the way through) comes out clean. Let the cupcakes cool for a few minutes before removing from the muffin tin.

Place softened butter into a large bowl or stand mixer. Beat the butter with a paddle attachment for 3-5 minutes or until very light and fluffy.

Add powdered sugar (start off with 1 cup). Beat for 2-3 minutes or until it becomes light and fluffy again.

Add sea salt, vanilla, nutella, and sifted cocoa powder. Beat until well combined and fluffy. Taste to see if you want to add more powdered sugar.

Add the heavy cream gradually, until the frosting loosens up and you get a silky smooth consistency. Don't beat for too long after adding cream or the frosting might curdle. Place the frosting into a piping bag with a piping tip.

Spread each cooled cupcake with a ½ tablespoon of Nutella. Flip the cupcake upside down and press the Nutella side into the chopped hazelnuts to coat lightly.

Frost each cupcake with a tower of Nutella buttercream. Sprinkle with leftover chopped hazelnuts. Top with either half or whole Ferrero Rocher chocolate.